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evaporator issues and technology whitepaper

Published October 31, 2018
Evaporator Issues and Technology
Evaporator Issues and Technology
Executive Summary
Evaporators are used to evaporate black liquor from approximately 15% dry solids to 70-85% dry solids.
As the dry solids percentage is raised, volatile compounds are released from the black liquor and separated
from the condensate, which then gets collected for reuse in the mill reducing the demand for incoming
water. Generally, the volatile compounds are incinerated in the gas form in kilns, boilers or stand-alone
incinerators. More recently, processes for producing sulfuric acid or low-odor methanol have been
introduced. Valmet has patented technologies for both processes, but neither will be covered in this paper.
Like all pulp mill unit operations, evaporators can have operating issues due to the processes occurring
inside the system or due to the complex mill chemistry. Almost all these issues fall into six main categories
including scaling, corrosion, liquor carry-over, vacuum issues, condensate segregation and foaming
Valmet's TUBEL concentrator addresses scaling, as does the mixing of ash, heavy black liquor (HBL)
recirculation and on-demand washing. Corrosion is addressed by using 304L SS and duplex alloys in
appropriate areas. Avoiding liquor carry-over is primarily a design issue relating to optimized vapor flow
and entrainment separators. A properly designed condenser will limit forcing velocities and help vacuum
equipment performance. Internal condensate treatment (ICT) and the effective use of the stripping
column will improve condensate segregation. Foaming is reduced by any of several means, and multiple
simultaneous control solutions can be used.
© Valmet
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Published October 31, 2018
Evaporator Issues and Technology
Evaporator technology focus
Traditionally, evaporators have been used to evaporate black liquor from approximately 15% dry solids
when they leave pulp washing to 70-85% dry solids before being combusted in the recovery boiler. As the
dry solids percentage is raised, volatile compounds are released from the black liquor and separated from
the condensate, and the condensate is then collected for further use. This maximizes reuse of condensates
in the mill and minimizes the environmental effect.
Modern evaporators are more complex and productive. They can maximize side streams that could then
be used as a commercial by-product such as methanol, tall oil and other substances. They can also process
other waste streams such as biosludge, chemi-thermomechanical pulping (CTMP) effluent, chlorine
dioxide (ClO2) generation effluent, etc. These capabilities allow mills to investigate how modern
evaporators can optimize mill-wide operations.
Evaporators fall into a few categories, based on how they operate: rising film (RF) evaporators, falling film
(FF) evaporators and forced circulation (FC) evaporators. In addition, all these types of evaporators can
be classified as a concentrator whenever they operate above the solubility limit for one of the various salt
species encountered in black liquor.
RF evaporators use vertical tubes where the liquor is introduced at the bottom, the tubes are steam heated
on the outside to the liquor boiling point, and partial evaporation occurs in the tubes. In the FF
evaporator, the liquor enters at the top and is distributed on a vertical heat exchange surface composed of
plates or tubes. As the film of liquor moves downward it is partially evaporated. FC evaporators are
tubular units that circulate enough liquor to flood the heat exchanger tubes which can be configured
either horizontally or vertically while using steam outside the tube to drive partial evaporation of the
liquor (FF or FC evaporators always provide the same high performance, while RF evaporators have poor
performance at low loads). Concentrators are typically based on the FF or FC concepts and are designed
to address the issues of high liquor viscosity and precipitation of components that have exceeded their
solubility limits (and thereby can produce scaling).
Figure 1. Problems inherent in the evaporator stage of the black liquor recovery process
© Valmet
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Published October 31, 2018
Evaporator Issues and Technology
As shown in Figure 1 (previous page), most operating issues in evaporators will fall into the following six
(6) categories. These include:
Liquor carry-over
Condensate segregation
Foaming liquor
Let's look at the solutions to these problems and technology that can enable these solutions, starting with
Black liquor is an aqueous solution of lignin residues, hemicellulose and inorganic chemicals used in the
cooking process. Based on typical dilution factors in pulp washing, the production of one ton of pulp
produces about seven tons of black liquor. In a kraft process, pulpwood is "digested" into paper pulp while
lignin, hemicelluloses and other extracts are removed as black liquor containing these components as well
as spent cooking chemicals. Major ionic species in the black liquor are carbonate (CO3), sulfate (SO4),
calcium (Ca) and sodium (Na). Virtually all the black liquor is burned in the recovery boiler, supplying all
the steam and much of the energy for electricity used by a typical mill.
As evaporation occurs, volatile components from the black liquor are released with the evaporated water
as part of increasing the dry solids percentage. During evaporation, as these inorganic salt compounds
reach their respective solubility limits, the liquor begins to supersaturate until it reaches what is known as
the "meta-stable limit", it then begins to crystallize and dissipate the chemical inequilibrium of
supersaturation, often very quickly. This formation of solid material from dissolved or ionic material will
often form scale on the inside of the evaporator. Also, once this has occurred, further evaporation will
constantly force additional dissolved salts to shift to the solid phase to maintain the solubility equilibrium.
Crystallization theory provides that the released supersaturation will either form new crystal nuclei for
other salts to deposit, will deposit on existing crystals or
nuclei in the flowing liquor or will deposit on other solid
surfaces. In the case of an evaporator this can be the tube
wall, vessel walls or pipe walls. Because evaporation units
that operate above the solubility limits are much more
likely to scale, these units are referred to as concentrators
rather than evaporators. All the evaporator suppliers
employ additional design features that better deal with
scaling. Scaling reduces the effectiveness of the
evaporation process.
Two predominate salt scales which form are burkeite
(which typically has a critical solids point at about 48-52%
dry solids) and sodium dicarbonate (which typically has a
critical solids point at above 55% dry solids).
TUBEL concentrator
TUBEL Concentrators (Figure 2) developed by Tampella,
now part of Valmet, were introduced in 1994 and address
the issue of scaling. An open design eliminates tube
plugging. Falling film liquor is evaporated on the outside
surface of the tubes by the steam/vapor on the inside. If
scaling occurs, the convex shape of the tube surface and
the open design of the unit make on-line washing easy.
© Valmet
Figure 2. The TUBEL concentrator addresses
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Published October 31, 2018
Evaporator Issues and Technology
There is normally no need for off-line washing and no TUBEL unit has required hydroblasting in Kraft
liquor applications.
TUBEL's mechanically rigid design handles the high steam temperatures and pressures that are
encountered in super-concentrators and reduces the risk for mechanical failures.
Valmet has delivered more than 110 TUBEL concentrators throughout the world, including North
America, South America, Australia, Europe and Asia. All these locations are producing high dry solids
content, several at 80% DS or above, without scaling problems.
Figure 3 shows Valmet's
evaporation technology
at Suzano Papel e
Celulose, Maranhão,
Brazil. The new cuttingedge XXL size, 6-effect
EVAPS plant includes a
concentrator and Effect
2 which use TUBEL
technology. The system
can produce heavy
liquor of 80% dry solids
using only low-pressure
steam at an evaporation
capacity of 1600 tons/h.
Figure 3. TUBEL technology at the Suzano Papel e Celulose evaporation line
Avoid operation at
critical solids
Figure 4 shows a
graph of the salt
concentration vs.
the percent dry
solids in the liquor.
The slightly
declining line
represents the
solubility limit
(Critical Solids
Point) for Na, CO3
and SO4. We would
prefer not to
operate the feed
section of the 1st
Effect at this salt
Figure 4. Recovery boiler ash mixing and heavy black liquor recirculation can avoid
operation at the critical solids point.
Remember from the
earlier discussion of
Crystallization Theory, released supersaturation will form new crystal nuclei, deposit on existing solid
salts or nuclei or deposit on other solid surfaces. When operating right at the critical solids point there are
© Valmet
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Evaporator Issues and Technology
no salt crystals or nuclei in the circulating liquor. Controlled mixing of recovery boiler ash and heavy
black liquor recirculation before getting to the critical solids point will bump the process past the
solubility limit and reduce severe scaling by ensuring there are "seed crystals" and nuclei in the circulating
liquor for newly released supersaturation to grow on. With this method, we can avoid the need to operate
the first section in the concentrator at the critical solids point.
Multiple sections for on-demand washing
With Valmet's concentrators, washing is fully automated (Figure 5), thus better controlling the removal
of scaling. Using controls supplied, the washing need is evaluated over time for each of the concentrator
sections. Whenever the need to wash is signaled (i.e. on demand), the operator initiates washing of one
individual section of the concentrator (i.e. washing only occurs when needed).
To execute the wash, weak black liquor is fed to the wash section lowering the circulating liquor
concentration to a target of 35% dry solids. Washing occurs by dissolving deposits back into the
circulating liquor. While this process begins once the solids are lowered below the critical solids point, the
target of 35% dry solids has been shown from experience to allow an effect wash in as little as 30 minutes.
During washing, the product liquor dry solids content of the concentrator is maintained. Both steam flow
and evaporation are continued in the section being washed. This ensures operational stability in both the
recovery boiler and evaporation processes.
Figure 5. CMPC Celulose Riograndense, Brazil has a completely new train with on‐demand washing and
REVAP Concentrator. Evaporation capacity: 430 tons/h, 75% DS product.
Normal vs. inverse solubility
Solubility is a function of temperature. Normally, salts increase in solubility with temperature. However,
some salts, such as calcium carbonate, become less soluble in water as temperature increases. This is called
inverse solubility.
Salts with normal solubility will foul cooling surfaces. Salts with inverse solubility will foul heating
surfaces. Since evaporation requires heat addition, the hottest portion of the liquor is immediately
adjacent to the heating surface. Inverse solubility salts can become supersaturated as the hot heating
surface lowers the solubility, causing solid salts to form near the tube wall. Controlling the differential
temperature to lower levels is critical in concentrator design to prevent this type of scaling.
© Valmet
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Published October 31, 2018
Evaporator Issues and Technology
As mills look to increase production and energy efficiency, they are typically running with higher Residual
Effective Alkali (REA) and at higher black liquor temperature and concentration in the recovery boiler.
This causes corrosion problems with traditional steel materials in the evaporators. As can be seen in
Figure 6 below, corrosion is now being seen in three main areas:
General thinning and stress corrosion cracking (SCC) in stainless-steel front-end bodies,
General corrosion and pitting in carbon steel (CS) intermediate bodies, and
Marine scale (barnacle type growth) with corresponding severe pitting in surface condensers.
Left unaddressed,
this corrosion can
be a significant
safety hazard as
metal failure could
cause the release of
hot caustic liquor,
potentially under
significant pressure
to the operating
areas around the
equipment. This
can also cause
unplanned outage
time. Even if the
corrosion doesn't
progress to the
point of metal
failure - it can limit
the evaporation
Figure 6. Three main areas of corrosion in evaporators. SCC in the SS concentrator
bodies, general corrosion and thinning in the CS back‐end bodies and marine scale in the
CS water side of the surface condenser.
An example of
corrosion limiting evaporation capacity can be found in a case study of an existing FC concentrator at a
Canadian mill. This concentrator vessel was supplied in 304L SS material with no corrosion allowance.
General corrosion throughout the entire vapor body occurred in the cone, top shell and bottom shell, with
several measurements below the minimum thickness. As a result, the vessel's vacuum capacity had to be
derated from full vacuum to -70 kPa. There was also stress corrosion cracking occurring in the heaters.
This forced the mill to adapt a maximum temperature limit of 145 °C on the operation of the
concentrator. These two restrictions reduced the overall delta-T for the train by 22% and as a result the
capacity of the overall evaporator system was decreased by about 15%.
To avoid tube thinning and stress corrosion cracking in concentrator units use duplex steel if operating
above the temperature threshold of 145 °C. Duplex stainless steels are called "duplex" because they have a
two-phase microstructure consisting of grains of ferritic and austenitic stainless steel. This ferritic
component provides both superior corrosion resistance and better resistance to SCC.
For CS back end effects, both shell and tube sides, use 304L stainless steel or duplex lite throughout.
For marine scale of the surface condenser water boxes, and the corresponding heavy pitting which will
usually occur behind the scale, use 304L stainless steel throughout.
© Valmet
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Published October 31, 2018
Evaporator Issues and Technology
Liquor carry-over
Liquor carry-over in the form of black liquor droplets carried through the evaporator system (also known
as entrainment) negatively impacts the process, resulting in more chemical loss, poorer condensate
quality (reducing the ability to reuse condensates and minimize freshwater usage) and increased corrosion
and erosion of metallic surfaces. Cost-effective evaporator systems require very low or negligible liquor
In older competing designs, a horizontal deck containing vertical flow chevrons occupied the annular area
formed between the outer diameter of the heater section and the outer diameter of the vapor body of the
To minimize liquor carry-over, Valmet applied Computational Flow Dynamic (CFD) modeling - typically
done on recovery boilers - to the vapor body section of the evaporator. Results of this modelling showed
that the flow of vapor should be symmetric about the vapor outlet between the primary and secondary
sections in the vapor body section of the evaporator. There should be a horizontal flow through the
chevrons in the secondary section. Lastly that primary droplet separation should occur before getting to
the chevrons. Figure 7 illustrates a properly designed vapor body section, which optimizes liquor droplet
Figure 7. Properly designed vapor body section with symmetric flow about the vapor outlet and horizontal
flow through the chevrons.
Valmet's optimized design also allows the vapor body diameter to be very close to the diameter of the
heater, which can be significantly smaller diameter than the older designs with vertical flow chevrons.
This reduced footprint saves on steel, concrete, piping and construction labor.
© Valmet
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Published October 31, 2018
Evaporator Issues and Technology
Vacuum equipment performance is important. Air leakage must be avoided. Operation at a poor vacuum
level lowers both the system capacity and the steam economy of the system. A vacuum pump or steam
ejector system allows the last effect in the evaporator to run at high vacuum (typically 25.4 in. Hg). These
devices remove the concentrated non-condensable gases (CNCG) that are released during evaporation. A
vacuum pump requires similar maintenance to other rotating equipment. While ejectors are less
maintenance intensive the erosion of the steam nozzle throat over time can cause a decrease in
performance, also scaling of any condensers supplied as part of the ejector system can also cause issues.
For either an ejector system or a vacuum pump to work properly the very large volume of water vapor
from the last evaporation effect must be condensed so that only saturated dry gas is handled by the
vacuum system. This is done in a shell and tube surface condenser. In the condenser, cool water circulates
inside the tubes and condenses vapor on the outside. Due to the high vacuum in the last effect, the vapor
has a large specific volume. For example, a pound of vapor may occupy 158 cubic feet of space at the
typical vacuum of 25.4 in Hg. This means there are extremely large volumetric flow loads on the shell side
of the condenser. Even in properly designed condensers, vapor flow velocities approaching 200 fps are not
uncommon. These high velocities can cause tube vibration and subsequent damage if the baffling on the
shell side and the expanded vapor plenum (larger diameter at the steam inlet allowing vapor to move up
or down prior to entering the tube bundle) are not designed properly. Valmet's design for a surface
condenser limits forcing frequencies of the tube and vapor velocities in the shell side to ensure damage
does not occur from tube vibration. In the example below Valmet used its design criteria to evaluate why
damage was occurring in an existing surface condenser.
Figure 8. This typical condenser (right) is operating well past the design limits (shown in left chart as
yellow, orange and red values). The result is vibration damage.
Figure 8 shows a side view of a typical condenser. Based on the shell side baffling and the inlet vapor
plenum the unit can be divided into flow zones (red letters). From zone to zone the flow area provided by
the baffling will dictate whether the flow is longitudinal, cross-flow or radial cross-flow. Then the
velocities flowing through each "flow window" from zone to zone can be compared to Valmet's standards
for that type of flow. In the chart in Figure 8, yellow, orange and red cells on the chart indicate flow
velocities that exceed Valmet's criteria for a proper design, with yellow being the least severe and red being
the most severe. This analysis successfully predicted areas where tubes were failing (i.e. corresponded to
the areas indicated by red and orange velocities).
© Valmet
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Published October 31, 2018
Evaporator Issues and Technology
Condensate segregation
As stated earlier, when evaporation occurs other volatile compounds are released. In evaporators these are
both TRS compounds such as di-methyl sulfide, di-methyl di-sulfide and methyl mercaptans as well as
alcohol compounds primarily methanol (MeOH) but with smaller amounts of higher carbon chain
compounds (ethanol, propanol, etc.). Under current environmental regulations a certain amount of these
potential hazardous air pollutants (HAPs) must be removed to avoid the need for further biological
wastewater treatment. Under this legislation, MeOH is also deemed a surrogate for all HAPs and will
condense with the evaporated water to form contaminated vapor condensate. Valmet classifies the vapor
condensate formed into three separate (or segregated) fractions based on the amount of MeOH present.
Condensate with very little MeOH is classified as A-Condensate (often referred to as clean condensate),
condensate with intermediate concentration of MeOH is classified as B-Condensate (often referred to as
contaminated condensate) and the condensate with high concentration of MeOH is classified as CCondensate (often referred to as foul condensate).
Untreated foul condensate is very odorous, cannot be reused in the mill and can't be released into the
environment. Collecting and stripping the foul condensates will allow the MeOH to be driven off and
burned in the vapor phase or condensed to liquid MeOH and used as a green fuel or as a chemical reagent
elsewhere in the mill. Stripped foul condensate becomes clean enough to be reused for brown stock
washing or as make-up water, thus reducing mill raw water demand.
Figure 9 shows Valmet's shell side design of
the back-end evaporator bodies and
potentially the surface condenser. This design
utilizes a circular baffle to create an outer and
inner section on the vapor side which are
optimized to concentrate as much of the
MeOH into as small a volume of condensate
as possible. This both increases the amount of
MeOH removed from the liquor to facilitate
mill compliance under environmental rules
and reduces further treatment costs for the
foul condensate whether via stripping or by
biological treatment.
Vapor from the previous effect is introduced
at the bottom on the shell side. As the vapor
rises, it flows counter-currently with the
Figure 9. Valmet's shell side design improves
condensate forming on the tube. This action
condensate segregation.
provides a stripping effect for the condensate
(research has shown this effect to be
equivalent to about 2.5 theoretical stripping
trays). Vapor enriched in methanol turns and condenses, flowing down into the inner section. A central
preheat pipe conveys the circulating liquor to a distribution plate in the top of the falling film units or is
arranged as the first pass (coldest) with cooling water in a surface condenser. This combination of
stripping in the outer section and sub-cooling on the inner section allows a large fraction (typically 8085% of the total condensate) of contaminated condensate to be collected at the bottom of the outside
section while a smaller fraction (remaining 15-20% of total) of foul condensate is collected from the inner
© Valmet
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Evaporator Issues and Technology
Internal condensate treatment (ICT)
Valmet has patented an internal condensate treatment (ICT) which utilizes the stripping effect discussed
above that occurs when vapor is arranged to flow counter-current to the condensate film flowing down
the outside of the tube. Using this method, a portion of B-Condensate is introduced onto the top shell side
baffle of the feed effect.
In figure 10 below, the back-end bodies are shown for a typical six-effect evaporator. This has normal
black liquor flow configuration for Valmet (4-5-6-3-2-1). Segregated shell sides as discussed in the
previous section are provided in the shell sides condensing the first three evaporations of the feed.
In this example with feed to
the 4th Effect, the evaporated
water and volatiles are
condensed in the 5th Effect
shell side. Evaporation in the
5th Effect is condensed on the
shell side of the 6th Effect and
evaporation from the 6th Effect
on the shell side of the surface
condenser. Three stages of
segregation are enough to
remove nearly all the MeOH
from the condensate.
Looking at an equilibrium
diagram for MeOH in water at
the conditions encountered in
the back-end effects, 80-85%
Figure 10. Valmet's ICT processes a portion of the B‐Condensate to make
of the MeOH is volatilized in
in the shell side of the 4th Effect.
each evaporation stage (i.e. if
you use 80%, 80% of the
incoming MeOH in the feed to the Fourth Effect is evaporated and 20% stays in the liquor to the 5th Effect.
In the 5th Effect, 80% of the remaining 20% is evaporated and only 4% goes to the 6th Effect. In the 6th
Effect, 80% of that remaining 4% goes to the shell side of the Condenser. Therefore, after three stages less
than 1% of the incoming MeOH remains in the liquor). Therefore, in evaporators there are almost always
at least two segregated shell sides and seldom more than three.
Returning to figure 10, a portion of B-Condensate is introduced on the top shell side baffle in the 4th
Effect and the nearly pure water vapor (remember less than 1% of the MEOH remained) is used as motive
fluid to provide the stripping effect described earlier. In this fashion, Valmet's patented ICT design
processes a portion of the B-Condensate to A-Condensate quality.
In summary Valmet optimizes the condensate segregation in the evaporator system by:
the shell side design in the backend bodies which maximize the MeOH content to the minimum
amount of C-Condensate and
the patented ICT design which processes a portion of the B-Condensate to A-Condensate quality.
This maximizes the amount of the highest quality condensate.
© Valmet
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Evaporator Issues and Technology
Condensate stripper
Foul condensates collected from the evaporators are combined with those formed from the digester. In
most mills a condensate stripper is used to strip MeOH from the condensate. If these systems are well
designed, the requirements for MeOH capture under environmental rules can be met. Regardless, the
load on the effluent treatment plant is reduced and odor from the mill is minimized.
A properly integrated stripper is essential for improving the steam economy of the mill. While nearly all
stripper suppliers will capture the overhead heat off the stripping column in reflux condensers that
preheat the black liquor flowing from effect to effect in the evaporator train, many of these installations
use live steam as the motive fluid in the stripper. Valmet integrates the stripper in the evaporators to use
vapor from the first effect as the driving force. This maximizes the steam economy of the mill and
prevents the loss of the live steam condensate when live steam is used. Condensate strippers usually
operate at an efficiency of 95% for methanol removal (higher for total reduced sulfur (TRS) compounds)
and upgrades the C-condensate to B-condensate quality or better.
CNCG opportunities
While not the focus of this paper, Valmet has two patented processes for producing additional revenue
from the CNCG released in the evaporator. CNCGs are removed from two places in the evaporator train.
Vapor from the stripper, or stripper off-gas (SOG) is taken from the shell sides of any stripping system
reflux condensers. This SOG is primarily MeOH with some water and TRS components. Today, several
mills condense the SOG from the top of the stripper column to produce liquid MeOH. Unfortunately, the
TRS compounds in this product make the liquid MeOH very malodorous. While the liquid MeOH is a
very stable high heating value fuel that can replace fossil fuel usage in the mill, the significant odor
prevents any possibility of selling the product and prohibits it from being used to make other higher value
products such as chlorine dioxide for use in bleaching. Valmet has a patented PuriMeth process to remove
the odor and produce a high value "green" product.
CNCG is also removed from the condensate hotwell. This CNCG is primarily TRS components and
Valmet has a process to produce high grade sulfuric acid from these gases.
For properly designing or servicing any of these types of systems, the ability to execute a methanol balance
is required. Surprisingly, not all service providers have this capability. Valmet has the software and
modelling capabilities to do these balances.
Foaming liquor
Foaming can be caused by fatty and resin acids in the incoming wood chips, which are converted during
the highly alkaline kraft process into sodium salts (otherwise known as tall oil soaps). The resulting salts
are surface active and readily form stable foam.
Foaming liquor during evaporation can create production disturbances and unplanned downtime as well
as increased chemical make-up and wastewater treatment costs. Foaming liquor is a function of dynamic
surface tension and is influenced primarily by extractives and the percentage of dry solids in the liquor.
Generally, higher extractive levels yield higher foaming tendency, while higher percent dry solids in the
liquor lowers the foaming tendency. Foaming can also be caused by liquor side air leaks.
Adding or releasing energy can also cause foaming. This can occur via: recirculation and liquor transfer
pumps, flashing liquor (liquor inlet above the body boiling temperature) and the initial boiling (on startup).
© Valmet
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Published October 31, 2018
Evaporator Issues and Technology
Solutions to mitigate foaming potential
To keep the level of extractives minimized, it is essential to remove soap from the weak liquor. This is
done by proper design in the weak liquor storage tank(s). Valmet has several methods to optimize soap
removal. One solution unique to Valmet is to distribute the incoming liquor more evenly across the entire
diameter of the tank. This allows for better up flow separation of the soap as it is less dense then the liquor
and floats.
There are two main ways to increase the percent dry solids of the liquor. Generally, if the circulating
liquor in a body exceeds 20% dry solids foaming will be controlled. Sweetening is the easiest control
option. This can be done by mixing the feed or circulating liquor with intermediate or even heavy liquor.
This has a negative result in that between the point the liquor recirculation is added and the point from
where it is taken, the hydraulic throughput increases by the amount of the recycled flow (increased power
consumption and, depending on the flow, potentially increased pipe sizes). You can also bypass liquor
around the effect. In a mass balance around the bypassed effect, a lower mass flow to the unit with the
same evaporation will cause the concentration of the circulating liquor to be higher. Higher solids
product then mixes with the bypassed liquor to provide the same feed solids to the next effect. This also
has a negative result in that the amount of MeOH that is driven off in the bypassed stage will be less and
an additional shell side of an effect may need to be segregated.
Air leakage can be addressed by regular and diligent maintenance checks of the vacuum effects using
ultra-sonic leak detection equipment. Also, in the original system design the use of gasketed connections
and body flanges, especially large ones, can be minimized.
Energy addition to the liquor by circulation or liquor transfer pumps can be addressed by designing the
system to have lower speed pumps. Where foaming is a concern, pump speeds should be minimized and
pump speeds of 1800 rpm or higher should be avoided where possible.
Energy release from foaming liquor can be mitigated by correctly designed flash nozzles which will both
slow the liquor and orient the liquor on top of an inverted cone which introduces a shear force to the
liquor to minimize foaming (similar to pouring beer down the side of a glass).
Foaming caused by the release of energy in the form of the first evaporation of the liquor (during startup) can be mitigated by starting up with higher percent dry solids liquor. For mills experiencing
significant foaming during start-up, one option is to fill with intermediate liquor rather than weak liquor
in their start-up procedures.
Even with good design, it is likely there will be some time during operation where the process
disturbances encountered may cause foaming. When foaming is occurring, some of the solutions above
can be applied in operations (i.e. liquor sweetening). Various anti-foam chemicals are available and can be
employed. These should be considered in the evaporation plant design, and automatic continuous
delivery systems should allow addition to at least the two weakest effects in the train or at flash tanks or
other vessels where foaming might be a concern.
© Valmet
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Published October 31, 2018
Evaporator Issues and Technology
Advanced control systems such as the Valmet Evaporation Optimizer can also mitigate foaming by
maintaining more even operation and a more consistent dry solids content profile over the evaporation
train by coordinating the feed, intermediate, and strong liquor dry solids controls.
Stack the deck to prevent liquor foaming. This doesn't have to be a fair game. Use as many mitigating
solutions as you can.
The problems inherent to evaporators in the black liquor recovery process include scaling, corrosion,
liquor carry-over, vacuum, condensate segregation and foaming liquor. Valmet's TUBEL concentrator
addresses scaling, as does the mixing of ash and HBL recirculation and use of on-demand washing.
Corrosion is addressed with 304L, duplex and duplex stainless steel in appropriate areas. Avoiding liquor
carry-over is primarily a design issue relating to optimized vapor flow. A properly designed condenser will
limit forcing velocities and help vacuum equipment performance. ICT and the effective use of the
stripping column will improve condensate segregation. Foaming is reduced by any of several means, and
multiple simultaneous control solutions can be used.
This white paper combines technical information obtained from Valmet personnel and published Valmet articles and
Valmet provides competitive technologies and services to the pulp, energy and paper industries. Valmet's pulp, paper
and power professionals specialize in processes, machinery, equipment, services, paper machine clothing and filter
fabrics. Our offering and experience cover the entire process life cycle including new production lines, rebuilds and
We are committed to moving our customers' performance forward.
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